1. Field of the Invention
The present invention relates to a delay time calculation apparatus for directivity control of a speaker array and relates to a delay time calculation method and a storage medium storing a program therefor.
2. Description of the Related Art
As a speaker array system including a speaker array having a plurality of speaker units, there may be mentioned a delay array type speaker array system (see, for example, Japanese Laid-open Patent Publication No. 2006-211230). In the speaker array system of this type, delay times of audio signals to be supplied to respective speaker units are adjusted to control a directivity characteristic of acoustic waves emitted from the speaker array. The directivity control is to control the propagating direction of a combined wavefront of acoustic waves output from the speaker units and control the degree of spread of the combined wavefront. In the directivity control disclosed in Japanese Laid-open Patent Publication No. 2006-211230, first delay processing for horizontal control is performed on an input audio signal IN10 to generate n first delayed audio signals corresponding to respective ones of speaker unit columns SP(i, 1), SP(i, 2), . . . SP(i, n) (i=1 to m). Next, second delay processing for vertical control is performed on respective ones of the n first delayed audio signals to obtain n×m second delayed audio signals, which are supplied to the speaker units SP(i, j) (i=1 to m, j=1 to n).
In an example technique to specify the propagating direction of a combined wavefront, the propagating direction is specified by vertical and horizontal steering angles. Assuming that a direction normal to an array plane of the speaker array is z axis, a vertical direction is y axis, and a horizontal direction perpendicular to the z and y axes is x axis, the propagating direction of the combined wavefront is specified by rotation angles from the z axis to the x axis and from the z axis to the y axis (horizontal and vertical steering angles). Thus, the propagating direction of the combined wavefront can be represented by a and β degrees by which the combined wavefront is steered leftward in the horizontal direction and downward in the vertical direction, making it easy to intuitively understand the propagation direction.
In the case of, for example, a speaker array having four speaker units SP(i, j) (i=1 to 2, j=1 to 2) arranged in two rows and two columns in the horizontal and vertical directions as shown in FIG. 7A, if the horizontal and vertical steering angles a and β are specified as shown in FIGS. 7B and 7C, a combined wavefront propagating in the direction represented by the two steering angles a and β can be generated by controlling delay time differences between audio signals supplied to the speaker units SP(i, j), as described below.
For speaker units disposed adjacently in the horizontal direction (e.g., speaker units SP(1,1) and SP(1,2)), audio signals are supplied that have a delay time difference therebetween corresponding to a difference between paths of acoustic waves output from these speaker units. For example, with reference to the audio signal for the speaker unit SP(1,1), the audio signal for the speaker unit SP(1, 2) is applied with a delay corresponding to a path difference (Dx sin a (see FIG. 7B)) relative to the speaker unit SP(1, 1). Similarly, forspeakerunits (e.g., SP(1,1) and SP(2, 1)) disposed adjacently in the vertical direction, the audio signal for the speaker unit SP(2,1) is applied with a delay corresponding to a path difference (Dy sin β (see FIG. 7C)) relative to the speaker unit SP(1, 1). Since the speaker unit SP(2,2) has path differences of Dy sin β and Dx sin a relative to the speaker units SP(1,2) and SP(1, 1), an audio signal with a delay corresponding to the sum of the path differences (Dx sin a+Dy sin β) is supplied to the speaker unit SP(2, 2).
With the directivity control specifying the propagating direction of a combined wavefront by horizontal and vertical steering angles and applying delays corresponding to path differences shown in FIGS. 7B and 7C, a problem is sometimes posed that some of the speaker units does not effectively contribute to the formation of the combined wavefront propagating in the direction specified by the steering angles. For example, in a case that relations of Dx=Dy=D and a=β=45° are satisfied in the speaker array in FIG. 7A, a delay for the speaker unit SP(2,2) becomes excessively large as compared to those for the speaker units SP(1,2) and SP(2,1) (see FIG. 7D), which causes the just-mentioned problem.